Tamoxifen is the most commonly used treatment for breast cancer [Jaiyesimi et al., Journal of Clinical Oncology 13:513-529 (1995)]. In addition, it is currently being considered for widespread use in healthy women for breast cancer prevention [Jordan, Proceedings of the Society for Experimental Biology & Medicine 208:144-149 (1995); Fisher et al., J.Natl.Canc.Inst. 90:1371-1388 (1998)]. Yet, despite its widespread use, its mechanisms of action remain obscure. Tamoxifen is a known estrogen receptor modulator that acts as an antagonist or partial-agonist. But it has also been reported to have many pleiotropic effects both in vivo and in vitro that cannot be explained by an interaction with the estrogen receptor [Kellen, Tamoxifen: Beyond the Antiestrogen, Birkhauser, Boston (1996)]. For example, tamoxifen has been shown to enhance drug sensitivity of multidrug resistant cells [Chatterjee and Harris, British Journal of Cancer 62:712-717 (1990); Berman et al., Blood 77:818-825 (1991); Berman et al., Leukemia 9:1631-1637 (1995); Weinlander et al., Journal of Cancer Research & Clinical Oncology 123:452-455 (1997); Altan et al., J.Exp.Med. 187: 1583-1598 (1998)], inhibit bone resorption and osteoporosis both in vivo and in vitro [Love et al., N.Engl.J.Med. 326:852-856 (1992)] and inhibit a number of channels, including the volume activated chloride channel [Zhang et al., Journal of Clinical Investigation 94:1690-1697 (1994); Ehring et al., J.Gen.Physiol. 104:1129-1161 (1994)] and calcium channels[Greenberg et al., Cancer Res. 47: 70-74 (1987); Song et al., Journal of Pharmacology & Experimental Therapeutics 277: 1444-1453 (1996); Williams et al., J.Biol.Chem. 271:12488-12495 (1996); Turner et al., Endocrinology 122:1146-1150 (1988)]. These effects have been attributed to inibition of P-glycoprotein [Callaghan and Higgins Br.J.Cancer 71:294-299 (1995)], calmodulin [Williams et al., J.Biol.Chem. 271:12488-12495 (1996)], and direct channel interaction [Zhang et al., Journal of Clinical Investigation 94:1690-1697 (1994)] respectively. Thus, the administration of tamoxifen as an estrogen receptor modulator results in a number of undesirable side-effects which are totally independent of the modulation of the estrogen receptor.
As mentioned above, tamoxifen has been shown to enhance drug sensitivity of multidrug resistant cells. Different drug-resistant cells overexpress a variety of membrane proteins including a subunit of a vacuolar H.sup.+ -ATPase [Ma, L et al., Biochem Biophys. Res. Commun., 182:675-681 (1992)], a protein with homology to CFTR [Cole et al., Science, 258:1650-1654 (1992)] and the P-glycoprotein, a 170-180 kD plasma membrane glycoprotein [Gottesman et al., Annu. Rev. Biochem., 62:385-427 (1993)]. The most generally accepted hypothesis for MDR suggests the P-glycoprotein uses ATP to power a molecular pump that removes chemotherapeutic molecules from the cell [Dano et al., Biochem Biophys., 323:466-483 (1973) and reviewed in Gottesman et al., Annu. Rev. Biochen., 62:385-427 (1993)]. This model proposes that chemotherapeutic agents diffuse down a concentration gradient into the cell and that the pump either transports the drugs out of the cytosol or serves as a flippase to expel them from the bilayer [Higgins et al., TIBS, 17:18-21 (1992)]. More recently it has been shown that changes of organelle pH associated with multidrug resistance (MDR) in tumor cells can lead to multi-drug resistance [see U.S. Pat. No. 5,851,789, Issued on Dec. 22, 1998, and U.S. application Ser. No. 09/080,739, filed May 18, 1998, the disclosures of which are each hereby incorporated by reference herein in their entireties] and that the effect of tamoxifen on MDR is related to the de-acidification of the cellular organelles.
Therefore, there is a need to identify modulators of the estrogen receptor that do not result in the same side-effects as tamoxifen. Thus, there is a need to determine the effects of tamoxifen that are independent of its role in modulation of the estrogen receptor. Furthermore, there is a need to identify the role tamoxifen plays in causing these effects. Finally, there is a need to identify assays for screening potential estrogen receptor modulators that can modulate the estrogen receptor as tamoxifen does, but do not cause the other undesirable effects.
The citation of any reference herein should not be construed as an admission that such reference is available as "Prior Art" to the instant application.